EVIDENCE OF RECENT GLACIATION IN ELYSIUM PLANITIA, MARS. J. Nussbaumer1, E. Hauber2, andR. Jaumann2, 1Department of Mineralogy, Natural History Museum, London, UK, 2Institute of Planetary Research,German Aerospace Center (DLR), Berlin, Germany.

Introduction: We present evidence for recent glacia-tion in southeastern Elysium Planitia, near the Martianequator. The surface features indicating glacial proc-esses include eskers, ground moraines, and bouldertrains. They are located next to wind-eroded, layeredmaterials, that are part of the Medusae Fossae Forma-tion. The characteristics of the esker-like features (con-certina eskers) suggest that they were formed by asurging ice-sheet. The identification of equatorial gla-ciation in the recent past might confirm theories aboutclimatic changes on Mars due to variations of theplanet´s orbital parameters.

Background: Calculations of spin axis precession,nutation, and orbit inclination reveal, that the obliquityand eccentricity of Mars have experienced dramaticvariations over time [e.g. 1, 2, 3]. Surface ice will bepreferentially deposited at locations with the lowestaverage annual temperatures and the highest saturationstate of the atmosphere. In periods of very high obliq-uity (~45°), ice may sublimate from the poles [4] andmay be stable in equatorial regions [5]. Obliquitychanges may be responsible for the observed stratigra-phy of the polar layered deposits [6]. The mapping ofvery young exogenic surface features like gullies [7, 8],polygonal ground [9], viscous flow features [10], dis-section of an ice-rich mantle deposit [11, 12] showsthat they are constricted to a latitude belt between ±30°and ±60°, indicating climatic control. In addition, evi-dence of recent cold-based glaciation has been foundon the northwestern flanks of the shield volcanoes ofTharsis (Arsia, Pavonis, and Ascraeus Montes) andOlympus Mons [13, 14, 15]. The neutron measure-ments by Mars Odyssey suggest a high hydrogen con-tent in the high latitudes that may indicate a water-richshallow subsurface in polar regions [e.g., 16, 17]. To-wards the equator, the depth to the ice-rich substrateincreases, in agreement with the theory that at presenttime (obliquity ~25°) the low and mid-latitudes experi-ence an epoch of ice sublimation, dessication, and sur-face degradation [18].

Glacial morphology in Elysium Planitia: In this sec-tion we describe the morphologic characteristics of thesurface features that we interpret as evidence for gla-cial processes. They include ground moraines, eskers,and boulder trains. The units are located in southeast-ern Elysium Planitia (Fig. 1) and were previouslymapped as members of the Medusae Fossae Formation(MFF) (units Amm and Aml of 19]. The MFF is an

Amazonian-aged deposit, the origin of which is enig-matic. It is widely eroded by wind and consist of vol-canic air-fall deposits [20].

Figure 1. General context map from Viking MDIM(250m/pxl), includes a high-resolution viking mosaic(14 m/pxl, marked by red line), ancient ice sheet ismarked by a blue line. Lower two images resemble apart of the image mosaic of Viking orbit 724A(14m/pixel; image width 60km, North is up, center at207W, -3S) and geomorphological sketch map. Hum-mocky ground-moraines, eskers, and possibly pingosare evidence for former glaciation. The outlined box Amarks the location of eskers deformed into a zigzagpattern typical for terrestrial concertina eskers.

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Detailed geomorphological mapping of high-resolutionViking Orbiter images (orbit 724A, 14 m pixel-1) re-veals a hummocky deposit, which resembles terrestrialground moraines. Hummocky ground moraines formby moving ice, whereas one side of the hummock iseroded by the ice. The debris is deposited on the lee-ward side of the hummocks. The average height of theglacial hummocks in Elysium Planitia is 30-40 m asobserved in MOLA data. It is bordered to the northeastby a topographical scarp. The high-standing depositsbeyond that scarp is wind-eroded, i.e., it is clearlymarked by yardangs. Its morphology indicates that it ispart of the MFF. The hummocky, ground moraine-likematerial is superposed by long and narrow ridges. Weinterpret these ridges as eskers. Eskers are sinuousridges of stratified glaciofluvial sand or gravel formedas infillings of ice-walled rivers and deposited in tun-nels in the ice [21, 22]. Locally, the sinuous pattern ofthe eskers (as seen in plan view) is changed into a jag-ged or zig-zag path. Icelandic eskers have been short-ened and crumpled into such a zig-zag pattern by anadvancing glacier snout [23; 24]. These so-called con-certina eskers are associated with surging glaciers,characterized by periodic changes in flow velocitiesover various timescales [25]. Increasing flow velocitiesdue to basal sliding occur only in wet-based glaciers. Itis important to note that the surges are not triggered byclimatic oscillations but by oscillations in the internalworking of the glacier [26]. Although it is tempting toto ascribe the former existence of a surging glacier onMars to previous climatic changes, this assumption isnot supported by terrestrial analogy. Terrestrial surgingglaciers are usually longer, broader, and less steep thannormal glaciers. They rather form on sedimentary thanon crystalline ground (note that this would also applyto the Elysium glaciers, since they are superposed onthe MFF). Boulder trains are widespread on the groundmoraine. While they also occur in the lee of obstaclesin large floods or downslope from outcrops on steepslopes, they can also be associated with glacial deposits[27]. Boulder trains in glacial environments indicateice movement during the last stage of glaciation [28].Some boulders are concentrically deposited aroundhills as a result of decreasing ice thickness (boulderrings, Fig. 1).

Conclusion: Several landforms in southeastern Ely-sium Planitia suggest a recent local or regional wet-based glaciation. The geographic location near theequator of Mars require, that they formed under differ-ent climatic conditions than today. A possible explana-tion is that they were formed during a period of veryhigh obliquity, when surface ice was stable at the Mar-tian equator [29]. A similar conclusion was drawn byHead and Marchant [30] for equatorial cold-based gla-

ciers in Tharsis. The difference between cold-basedglaciation in Tharsis and wet-based glaciation in Ely-sium Planitia might be explained by the higher eleva-tion and, therefore, colder temperatures under whichthe Tharsis glaciers formed. Our notion of ice in Ely-sium Planitia is in agreement with previous reports ofglaciation in Elysium [31]. It also supports observa-tions of shallow equatorial ground ice on Mars in re-cent times as inferred from young rootless cones inter-preted as pseudo-volcanoes [32].

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